1. Technical Field
The described invention relates generally to controlling dose uniformity during ion implantation. More particularly, the described invention is directed to controlling the uniformity and dose of a semiconductor wafer by using variable scan velocity in multiple scan directions and is particularly suited for low energy implant applications.
2. Related Art
Ion implantation processes typically require a uniform and consistent dose or amount of ions to be implanted into a semiconductor wafer. Dose is generally a function of ion beam current density and time that the wafer spends in front of an ion beam. Current serial implanters provide an ion beam that is horizontally either an electro-statically scanned spot beam or a uniform ribbon beam. Serial implanters may also use a magnetically scanned spot beam, and a dual mechanically scanned (raster) spot beam. One conventional approach provides a horizontally uniform ion beam, and then mechanically moves the wafer at a constant velocity in the vertical direction. In another conventional approach, the wafer is moved vertically and the ion beam is moved back-and-forth across the wafer. Unfortunately, both of these approaches are problematic for the low energy market because the beam has to be manipulated, which requires continual tuning of the ion beam. As a result, the required implant time is increased and wafer throughput is decreased.
One approach to address this situation is disclosed in U.S. Pat. No. 6,677,599 to Berrian. Under this approach, a wafer is translated at a non-uniform velocity through the ion beam as it is simultaneously rotated at a rotational velocity. A shortcoming of this device is that the constant rotation of the wafer introduces unnecessary complexity into attaining a uniform dose. Constant rotation also introduces two other problems. First, maintaining constant tilt and wafer orientation is very difficult. In particular, the wafer holder needs to rotate around its axis for any tilted implant, which greatly complicates the mechanism. In addition, the single tilt axis provided by Berrian does not provide adequate exposure to the different crystal axes of the wafer because it only tilts about a single axis. Second, continuous rotation of a product wafer has been shown to potentially damage the product wafer a couple of different ways. First, fine scale structures on the wafer may not have sufficient structural integrity to withstand the centripetal acceleration, and, second, the rotation greatly adds to the kinetic energy when particles collide with the wafer surface and enhance the destructive potential of the particles. That being said, certain applications for continuous rotation exist.
In view of the foregoing, an approach is desired for allowing the use of an at least partially untuned beam to be used to attain a uniform implant without the problems of the related art. In addition, there is a need in the art for an approach to using continuous rotation and provide additional tilt capabilities.